The goal of these studies is to understand the structure, the function, and the genomic organization of immunoglobulin heavy and light chain gene families in a primitive vertebrate so as to provide new insight and generate new hypotheses fundamental to understanding immunoglobulin gene organization and function in man. With completed studies establishing the catfish as a phhylogenetically relevant model system to study immunoglobulin gene organization and function, five specific aims are proposed: 1. Two classes of catfish light (L) chain genes have evolved within the genomic framework of a clustered gene arrangement. Significant changes regarding VL and JL diversity have occurred. Chief amongst these is the divergence of VL kappa families. The diversity expressed in different V kappa families will be determined. 2. Genomic studies of L chain genes will be done at three levels: A) fluorescence in situ hybridization on metaphase chromosomes to define their location B) pulsed field gel electrophoresis to determine the size of the loci and patterns of cluster organization, and c) long-range Pl genomic cloning to study the gene organization and structure. 3. A representative panel of B-cell lines will be developed, and the L chain genes which are expressed will be characterized to determine if the principles of allelic exclusion are operational. 4. VH genes evolved within two different locations within the compact catfish IgH locus: one which is upstream and separated from the second which is downstream and closely linked with DH and JH segments. Studies into genomic organization and function of interspersed VH gene families will provide new insight into VH repertoire expression. The genomic organization of VH genes will be further characterized by two approaches a) pulsed field gel electrophoresis and b) long range genomic cloning to define the organization and structure of VH gene segments within distal and proximal regions. 5. These studies will analyze the expression of the seven VH families during different developmental stages. In addition, sterile VH transcripts and VDJ rearrangements representing these VH families will be characterized to determine which VH, DH, and JH segments are expressed. These analyses will determine the patterns of gene utilization, the junctional diversity within the encoded CDR3 region, and the interrelationships of genomic organization and developmental expression.